This invention relates to a new method of a readout for a 1, 2, or 3-dimensional memory device. Information is stored in magnetic memories by orientating the magnetization of ferromagnetic units like cores, or magnetic powder particles. Typically, a core, which is a ring shaped magnet, may be magnetized clockwise or anticlockwise. Each state represents a given unit like a "zero" or "one", or a "plus" or "minus". A "readout" device makes it possible to determine the magnetic orientation of these magnetic units. This can be done electromagnetically, as e.g. in the case of magnetic cores, where a change in the magnetization of a core leads to an induced voltage in a wire going through the center of the core. The magnetic orientation of magnetic particles can be determined also optically, since the orientation of the polarization plane of a polarized light beam will be rotated if it interacts with a magnetic material. The rotation can be quite large, if light is sent in transmission through a ferromagnetic or ferrimagnetic material. This is the Faraday effect, where rotation of 10.sup.3 to 10.sup.5 deg/cm can be found in materials like iron, or cobalt and in compounds like BiMn. This has been discussed in detail by J. F. Dillon, Journal of Appl. Physics, Vol 39, p. 922, 1968. The reflection of polarized light on ferromagnetic surfaces leads again to a rotation of the orientation of the polarization plane. This is the Kerr-effect. Typical rotations are less than 1 deg. In BiMn, values of about 1 deg. have been found.
It has been shown recently (Klaus Schroder, "Selective Dipole Orientation of Individual Volume Elements of a Solid Body," U.S. Pat. No. 3,890,604), that one can construct a 3-dimensional memory, in which "bits of information" are stored by superimposing ultrasonic pulses and a magnetic field in such a way that only one selected volume element has a higher stress than the rest of the material. The applied field H was selected in such a way that only in the highest stressed section the applied field was large enough to change the magnetization direction. This made it possible to "write" a bit of information into the magnetic memory. One would "read" if the magnetization of a magnetic particle was lined up in a given direction by superimposing again in the same time sequence the same set of ultrasonic pulses and the magnetic field used to "write". If the field H was parallel to the magnetic orientation, a zero average voltage would be induced in a sense coil surrounding the sample. However, a change in the magnetization direction of the particle due to the fact that it was antiparallel aligned to the field H and now changed to a magnetization direction parallel to H leads to an induced voltage in the sense coil surrounding the sample. Similarly to the memory with magnetic particles, one can use a memory with ferroelectric particles in a non-ferroelectric matrix. This system has been described in detail in the patent "K. Schroder `Selective Dipole Orientations of Individual Volume Elements in a Solid Body` U.S. Pat. No. 3,890,604".